The Low Electronic States of Simple Heteropolar Diatomic Molecules. I. General Survey

The three lowest energy states of molecules ${\mathrm{H}}_{2}$, ${\mathrm{M}}_{2}$, and MH are here deemed more or less analogous and labeled $N$, $T$, $V$; these are respectively of the types $^{1}\ensuremath{\Sigma}^{+}$, $^{3}\ensuremath{\Sigma}^{+}$, $^{1}\ensuremath{\Sigma}^{+}$. States $N$, $T$ and $V$ must exist also in all other uni-univalent molecules, e.g., ${\mathrm{X}}_{2}$, XY, HX, AgX, MX (M = alkali metal atom, X or Y = halogen atom). The characteristics of the states $N$, $T$, $V$ (polarity, ionicness, etc.) are surveyed. Only in ${\mathrm{H}}_{2}$ is state $T$ yet known empirically; $N$ is the normal state always; $V$ is to be identified with a state known from band spectra in the case of most molecules of the types above named. In molecules containing one halogen atom, there should be an additional low energy group of states (types $^{3}\mathrm{II}$, $^{1}\mathrm{II}$), here labeled the $Q$ group. Various points connected with the approximation of polar diatomic molecule wave functions using atomic orbitals, and using molecular orbitals, are discussed. Difficulties and arbitrariness inherent in the definition of polarity and ionicness are examined.